Method and burner for rotary kilns

ABSTRACT

The present invention relates to a method of and a burner for generating a flame in a combustion zone of a rotary kiln. The burner comprises at least a burner tube ( 4 ) extending into the kiln from outside of the kiln and means ( 3 ) for introducing fuel into the burner tube and means for leading primary air via the burner tube into the combustion zone of the kiln. A characteristic feature of the invention is that the burner is connected to a gas turbine ( 1, 2 ) via a connecting tube ( 7 ) for leading flue gas generated in the gas turbine into the burner tube as primary air.

The present invention relates to a method of and a burner for generatinga flame by means of the burner in a combustion zone of a rotary kiln.

Rotary kilns are typically used for treating various solid substances,especially when the treatment requires high temperature. Also typically,the treatment processes are endothermic, i.e. they require introductionof external heat into the kiln from outside of the kiln. Some examplesof this kind of processes are e.g. reduction of oxidized ores and oxidedconcentrates and calcination of various compounds, such as combustion ofclinker and lime. Treated material exiting the kiln is often hot and inorder to improve heat economy, the heat therein is recovered e.g. bymeans of preheating the combustion air being introduced into the kiln.

Heat sources utilized in the kilns include liquid, gaseous and solidfuels, such as oil, natural gas and carbon dust. The burner is attachedto the hot end of the kiln. Usually the burner has the construction of amulti-passage tube introduced through the end of the kiln via an openingarranged therein. The discharge end of the burner extends into the kilnto a location, which is optimal in view of both fuel combustion and heattransfer, which location depends on requirements set by the processpracticed in the kiln. In some cases, the burner tube may extend only tothe level of the inner surface of the burner end, but it may also extendseveral meters into the kiln. The burner tube is provided with passagesfor fuel (fuels) and combustion air, possibly also for additivesnecessary for the operation of the process.

Especially in treatment processes producing a hot product (clinker,lime, so-called lime sludge), the heat therein is recovered bytransferring it into combustion air required in the firing of the fuelused in the process. In such a case, this air (so-called secondary air)is usually directed into the kiln by-passing the burner, and onlyso-called primary air is directed through the burner, which primary airis necessary for igniting, stabilizing (maintaining a constant ignitionpoint) and forming of the flame. The proportion of primary air variesdepending on individual burners and applications, but most typically itis 10-40% of the total volume of combustion air. The primary air isdirected to the burner in order to ensure controlled ignition of thefuel and a constant ignition point (stabilizing of the flame) and toachieve a controlled form of the flame in the kiln. The primary air isled to the burner via a fan of its own.

However, the present primary air arrangements do not always provide forthe desired results in view of both flame control and heat economy ofthe kiln. Moreover, the ever more exacting environmental requirementsset increasingly tight limits to nitrogen oxides emissions. For example,reducing the amount of primary air typically results in a decrease innitrogen oxide emissions, but at the same time complicates controllingthe form of the flame, as well as adjusting the center of combustion.These, in turn, are factors, which have an effect on e.g. the heateconomy of the process. The object of the present invention is toprovide a method and a burner for more efficient controlling ofcombustion in a rotary kiln, such as a lime kiln, at the same timeresulting in decreased detrimental emissions, e.g. nitrogen oxideemissions, compared to prior art systems.

The characteristic features of the present invention are disclosed inthe appended claims. The invention is essentially based on the use offlue gas from a gas turbine instead of air as the source of primary air.Thus, a primary air fan has been replaced by a gas turbine.

In known burners, primary air is introduced at an overpressure of a fewkPa, which primary air is un-preheated or slightly preheated, typicallyhaving a temperature of e.g. 150-200° C. Air is known to contain oxygenin the amount of about 21% of its volume. In the new burner, gas exitingthe turbine and entering the burner tube most often has an oxygencontent of 15-16% and a temperature of 400-800° C., depending on thecapacity of the turbine and pressure loss of the burner tube.

The object of the exhaust gas of the gas turbine is just the same as theobject of air introduced by means of a primary air fan, but in theburner arrangement according to the invention the amount of airintroduced to the ignition is clearly smaller than in known burners andwith smaller flow volumes of oxygen and gas, typically only 4-10% of thetotal amount of combustion air. The fuel flow required by a gas turbineis very small compared to the main fuel flow, usually only a fewpercents.

A characteristic feature of the burner is that multiple various fuelsmay be burned therein simultaneously, even if they represent all threeforms, e.g. solid, liquid and gaseous forms.

The invention may preferably be applied in lime sludge kilns, lime kilnsand cement kilns.

Other air required in the kiln in addition to primary, such as secondaryair, bypasses the burner. Typically the secondary air is heated bycausing it to contact with the material combusted in the kiln.

The present invention is explained in more detail with reference to theappended Figures, in which

FIG. 1 represents a preferred burner arrangement according to theinvention, and

FIGS. 2 a and 2 b represent a second preferred burner arrangementaccording to the invention.

The construction and implementation principle of the burner isillustrated in FIG. 1. The burner is formed of a tube 4, which extendsinto the kiln via an opening in the end wall 8 of the kiln. Exhaust gasfrom a gas turbine, i.e. primary air is led to the discharge end of theburner via the burner tube. Fuel, e.g. heavy oil, may be introducedconventionally by leading it from line 12 by means of a tube 3 (burnerlance) of its own into a nozzle located in the discharge end 13 of theburner tube 4. A conventional embodiment comprises arranging itconcentrically inside the burner tube so that it is surrounded byprimary air, but other construction solutions are also possible.Depending on the quality of the fuel, it may also be fed into theforward end of the burner tube, in which case it will be mixed intoprimary air flowing in the tube and inflame in the formed mixture.

According to the invention, a gas turbine is connected to the burner,said gas turbine comprising a compressor 1, wherein air is led and acombustion chamber 2 and turbine 11 connected thereto. Fuel in line 9,such as natural gas or oil, and air from the compressor, are led intothe combustion chamber 2, the flue gases (i.e. primary air) from whichcombustion chamber are led via the turbine 11 rotating the compressor.The power requirement of the compressor 1 from the turbine 11 forgenerating the pressure needed in the combustion chamber is so smallthat the temperature decrease of the gas in the turbine is usually only50-100° C.

A characteristic feature of the burner arrangement is that gas (primaryair) generated in the combustion chamber 2 and exiting the gas turbineis fed via a short connecting tube 7 into the actual burner tube 4. Theconnecting tube 7 is most suitably constructed so that it is connectedto the burner tube 4 outside the burner end 8 of the kiln.

The gas turbine unit with its combustion chamber is relatively lightweighted. It may be positioned separate from the burner tube, ifdesired, but preferably the burner tube, gas turbine unit and theconnecting tube between them are integrated so that the gas turbine unitis supported to the burner tube via the connecting tube and, if needed,additional supports. An advantage of this kind of unit formed of the gasturbine and burner tube connected together is that its position inrelation to the kiln may be changed. This also has an effect on theoperation of the kiln: The burner tube is not always located in thedirection of the longitudinal axis of the kiln, but it is typicallyinclined in the direction of the material bed to be treated, in order tointensify heat transfer from the flame to the bed. A fixed connection ispreferable also constructionally, as the connection between the gasturbine and the burner tube is effected with a stationary connectingtube instead of using a flexible hose, which has to stand temperaturesup to 800 degrees of Celsius, when necessary. A possibly needed coolerfan for the burner may be connected to the burner tube in acorresponding way.

According to FIG. 1, the gas from the turbine is fed into the burnertube 4 via an inclined connecting tube 7. In principle, the gas may befed either tangentially from the side of the burner or axially via theend of the burner. The gas pressure loss in the burner tube (backpressure of the gas turbine) depends on the feed direction of the gas,so that the least loss is obtained via axial feed and greatest viatangential feed, thus the optimal construction has to be decided foreach case individually.

FIGS. 2 a and 2 b represent a burner arrangement, in which the gas fromthe gas turbine is led into the burner tangentially. In accordance withFIG. 2 a the burner comprises a burner lance 23, a casing tube 30 forthe burner lance, if needed, a burner tube 24 and a cooling air housing25. In this embodiment the burner tube 24 comprises a cyclone part 32,which is connected to the straight part 24 of the burner tube via a cone26. Fuel is fed into the burner lance 23 from line 33. The gas from thegas turbine is introduced into the cyclone part 32 via a connecting tube27, which connects the burner tube and the gas turbine and is attachedtangentially to the cyclone part 32. The end wall of the kiln is markedwith reference numeral 28.

FIG. 2 b shows as a cross sectional view via line A-A of FIG. 2 a theconnection of the burner tube to the gas turbine. The gas turbinecomprises a compressor 21, a combustion chamber 22 and a turbine 31.From the gas turbine the gas is led into the cyclone part 32 of theburner tube via connection tube 27, which is tangentially connected tothe cyclone 32. Fuel is introduced into the combustion chamber via line29.

The amount of ignition energy at the discharge end of the burner may beincreased, if needed, by means of so-called intermediate combustion.Normally the burner tube is dimensioned so that the fuel fed thereincannot bum in the tube, but inflames only when the mixture is dischargedfrom the burner into the kiln. Intermediate combustion is enabled byproviding the burner tube with a zone, in which the flow speed ofprimary air is reduced to be lower than the propagation speed of theflame front by locally increasing the cross-sectional flow area ofprimary air. A preferred method of implementing intermediate combustionis to arrange the zone in the front end of the burner tube and led theexhaust gas from the gas turbine into the burner tube tangentially sothat a cyclone-shaped intermediate burner is formed in the front end ofthe burner tube, as shown in FIGS. 2 a and 2 b. This way, thetemperature of the gas may be increased to be even more than 1000degrees centigrade, if necessary. The fuel necessary for the temperatureincrease is usually fed into the connecting tube 7 between the gasturbine and the burner tube via line 10 in FIG. 1 and into theconnecting tube 27 in FIG. 2 b. The space required for intermediatecombustion does not necessarily need to be located in the front end ofthe burner tube, but may be arranged in another location therein.

As the exhaust gas from the gas turbine has a temperature of severalhundred degrees (400-800° C.), the portion of the burner located insidethe kiln tends to become hotter than when using cooler primary air. Forthis reason, in the arrangement according to the invention, the burnertube is preferably cooled. According to the principal constructionillustrated in the figures, the burner is provided with a concentricalouter housing 5 and cooling air is introduced between the housing andthe actual burner tube 4 by means of a fan 6, which air exits via anannular slot between the tubes into the kiln (flame). A typical amountof cooling air is only 1-3% of the total combustion air flow. Inindividual objects, thermal insulation around the burner tube may beprovided for increased protection.

By means of a burner according to the invention, the nitrogen oxidelevel can be reduced compared to using burners operating with air. Themost important way to minimize the emission level is considered to bedecreasing the amount of primary air (primary oxygen) and fastening thetemperature increase in the flame after ignition, due to increasedamount of ignition energy Fast burning results in oxygen deficit in theflame and the combustion zone of the kiln, due to which thermal NO ismostly generated via OH radicals, which react to NO remarkably slowerthan free oxygen. The oxidation of nitrogen contained in the fuel to NOreduces as the oxygen content decreases, while the reduction of NO tomolecular nitrogen increases.

Compared to present rotary kiln burners, the new solution also providesfor better controllability of the flame in view of both the form of theflame and the rate of combustion. The latter is regulated by thecapacity of the gas turbine, which affects the volume of exhaust gasflow from the turbine and the temperature of the flow. The combustionvelocity also has an effect on the height of the flame and the burningtemperature, and further the heat transfer from the flame to thematerial being processed in the kiln.

The burner also provides for a larger power adjustment zone than presentrotary kiln burners. Stable combustion is possible even at very lowpower, because an amount of energy corresponding to the full capacity ofthe gas turbine may in the best case be introduced into the burner asignition energy, simultaneously maintaining the main feed of fuel at avery low level without causing the burner to go out.

1. A method of generating a flame in a combustion zone of a rotary kilnby means of a burner comprising at least a burner tube extending intothe kiln from outside of the kiln and means for introducing fuel intothe burner tube and means for leading primary air via the burner tubeinto the combustion zone of the kiln, characterized in that flue gasgenerated in a gas turbine connected to the burner is used as primaryair.
 2. A method according to claim 1, characterized in that thetemperature of the flue gas of the turbine is 400-800° C.
 3. A methodaccording to claim 1, characterized in that the fuel is fed into thedischarge end of the burner tube.
 4. A method according to claim 1,characterized in that the fuel is fed into the front end of the burnertube, wherein it is mixed with primary air entering from the gasturbine.
 5. A method according to claim 1, characterized in that theprimary air from the gas turbine is fed via a connecting tube so thatprimary air is fed into the burner tube tangentially and that fuel isfed into the connecting tube, whereby a cyclone-shaped intermediateburner is formed in the burner tube.
 6. A burner for generating a flamein a combustion zone of a rotary kiln, said burner comprising at least aburner tube (4) extending into the kiln from outside the kiln and means(3) for feeding fuel into the burner tube and means for leading primaryair via the burner tube into the combustion zone of the kiln,characterized in that the burner is connected to a gas turbine (1,2) viaa connecting tube (7) for leading flue gas generated in the gas turbineinto the burner tube as primary air.
 7. A burner according to claim 6,characterized in that the connecting tube (7) is inclined in relation tothe burner tube.
 8. A burner according to claim 6, characterized in thatthe connecting tube (7) is positioned axially in relation to the burnertube.
 9. A burner according to claim 6, characterized in that theconnecting tube (7) is tangential in relation to the burner tube.
 10. Aburner according to claim 6, characterized in that the burner furthercomprises means for introducing fuel into the connecting tube forincreasing the temperature in the burner tube.
 11. A burner according toclaim 6, characterized in that the burner tube (4), connecting tube (7)and gas turbine (1,2, 11) are constructed as a unit, the position ofwhich in relation to the kiln is adjustable.
 12. A burner according toclaim 11, characterized in that said unit further comprises a coolingair fan (6).